Preliminary Designs & Prototyping

Initial Design Considerations

To begin the design process, the size of robot had to be decided. Matching the true scale of the adult oceanic manta ray was not an attainable goal since they generally achieve a 240in wingspan. Therefore, a small scale version with a wingspan of 66in was agreed upon. Using anatomical ratios shown in Figure 3.1.1, a body length of 33in was calculated. These dimensions led to a rigid body structure that is approximately 19in wide.

Anatomical Ratios

The central “body” provided several necessary features for the robot. The first was acting as a watertight housing for the batteries, electronics, pumps, and valves that were be necessary for the robot’s operation. The body is also a rigid structure that acts as the ground for the flapping mechanisms. The structure was initially designed in two pieces made up of a bottom tub and a detachable cover with a waterproof seal between the sections. This configuration would allow for easy access for internal assembly, maintenance, and battery exchange/charging. Initial considerations for building the chassis structure consisted of rapid prototyping, milling metal, or injection molding plastic.

The initial design included the fins directly attached to each side of the rigid chassis structure. There were many options that needed to be considered, necessitating the implementation of some small prototypes, before the decision could be made for the hull structure and design of the fins.

Actuation Comparison

Actuation was highly important to the success of the project. Being able to achieve a functional range of motion with a reliable mechanism was paramount in achieving the project’s goals. Desired actuation was based on the nature of the movement of actual manta rays. Achieving a similar magnitude of fin deflection, about 35 degrees, would provide the closest representation of the manta’s motion. Additionally, the design of the sub-systems depended on the method of actuation. Considerations for several types of actuation are explained in more detail below.

Electroactive Polymer Actuation

Electroactive Polymer

Artificial muscles refers to use of an electroactive polymer that contracts when electricity is passed through it. One benefit to this method is that the actuation and assembly are one unit. This means the artificial muscle will reach the length of the fin as well as provide the mechanical force to move the fin. This combination results in fewer parts required for the system to function. However, this technology is still relatively new and is prohibitively expensive. Furthermore, other biomimetic projects have cited the electroactive polymers as having a limited range of motion compared to other actuation systems.

Conventional Hydraulic Actuation

Fluid Actuation System

Fluid driven systems are often used in soft robotics. These systems involve pressurization of a cylinder to move a piston, which applies a force. These systems are generally very reliable and offer uniform loading. Additionally, pumps required for a fluid driven system can be selected for energy efficiency without sacrificing much function in the actuation system. However, due to limited space for the piston to move based on the cylinder’s length, a fluidic system would have significantly limited range of motion.

Geometry Driven Actuation

Geometry Driven Actuation

Geometry driven systems use linkages and geometric structures to transfer loads. This allows for wide range of motion and complex motion depending on the configuration of the system. Although these systems are versatile, they can become very complex, making design and fabrication difficult. Additionally, many large geometry driven systems require multiple motors per assembly, making the energy cost relatively high.

Wire Driven Actuation

Wire Driven Actuation Cross Section

Wire driven systems use inextensible cables to transfer force along the length of an assembly. These can be paired to offer directional control of the contraction on either side of an assembly, mimicking muscles. Due to space limitations, wire driven systems in submersibles have very limited motion, generally yielding basic two dimensional motion.

PneuNet Actuation

PneuNet Actuation with Restricted Motion

A pneumatic network (PneuNet) actuation system combines the concepts of geometry actuation and fluid driven actuation, allowing the pressurization of tubes or channels to work with geometric structures, yielding a hybrid method of actuation. This system allows a wide range of complex motion due to the ability of the geometric system to bend in multiple directions, and the ability of the fluid system to create equalized pressure throughout the actuation tubes. Fluid systems also use less energy than a mechanical system would, due to the reduced number of actuation devices needed per assembly.

Preliminary Testing

In the decision matrix below, several actuation methods were scored according to whether they hindered, did not meet, met, or exceeded relevant criteria. Based on the results of that decision matrix, two types of actuation were tested for possible implementation. The first was a fiber-reinforced elastomeric tube, derived from the work done by Bishop-Mosher et al. This method was able to produce complex motions including bending and torsion, however, it was difficult to fabricate. The second method involved molding channels directly into the silicone fins. Rows of channels were molded along the top and a row along the bottom of the fin. The flapping motion was created by pressurizing either the top or bottom row of channels.

 

Weight

Artificial Muscles

Fluid Driven System

Geometry Driven System

Wire Driven System

Combination of Fluid and Geometry Driven System

Capable of Complex Motion

2

1

1

1

0

2

Range of Motion

2

0

1

1

0

2

Energy Efficient

2

2

1

1

0

1

Reliable (low failure rate)

1.5

2

1

2

1

1

Cost Effective

1

-1

1

1

2

1

Low Risk of Leaking

1

2

1

2

2

1

Proven Results

1

0

1

2

1

2

Unweighted Totals

6

7

10

6

10

Weighted Totals

10

10.5

14

6.5

15.5

Key: -1 - hinders criteria; 0 - does not meet criteria; 1 - meets criteria; 2 - exceeds criteria

Propulsion System Decision Matrix

Combinations of Fluid and Geometry Driven Systems

Fiber Reinforced Elastomeric Tube System

Fiber reinforced elastomeric tube actuation relies on the interaction between the expansion of the tube and the rigidity of the fiber surrounding it. As the tube expands horizontally, the fibers limit the movement of the tube in certain directions based on how it is configured. With proper design, exact motion can be achieved with this system. Combining multiple tubes yields a larger range of complex motion. Multiple systems of these tubes in a sequence would theoretically have allowed for the motion desired for a manta ray fin.

Multi-Elastomer Actuation System

To implement fiber-reinforced elastomeric tubes, they would have needed to be pre-fabricated. The silicone fins would then be poured around the tubes. This guarantees the tubes to be perfectly fitted inside the fin, allowing all generated forces to be applied directly to the fin.

Several of these actuators were fabricated using latex tubes wrapped with different configurations of sewing thread that were bonded using rubber cement. These tubes were difficult and time consuming to construct because the threads needed to be placed precisely. The prototypes were tested by pressurizing the tube with air from a bike pump. It was found that the prototypes had very limited and erratic motion. In addition, there were problems effectively sealing the free end of the tubes. Ultimately, the complexity in the fiber placement, the length of the process and low output deflection made it impractical to move forward with this design.

Silicone Pneumatic Network Bending System

After the failures with the fiber reinforced elastomeric tube actuators, some more research was done for the pneumatic network bending system. One excellent resource was the Soft Robotics Toolkit, a well-respected Harvard research website. This included many small scale example actuators that used the pneumatic network method, including step-by-step instructions for manufacturing them. The WPI Soft Robotic Fish paper was another useful resource that implemented this type of molded silicone actuator to create a successful biomimetic robot that mirrored the flexibility and movement of a fish. Due to the similarities in application to the manta ray, this became the chosen type of actuator to move forward with in prototyping.

 

Soft Robotic Fish Actuation

Small Scale Fin Prototypes

There were several iterations of small scale fin prototypes consisting of different designs and materials to determine which combination would be the best option for scaled prototyping and, ultimately, full sized fins. Some simple simulations were conducted in SolidWorks to try to predict the response. Later, these simulations were compared to the results of the small prototype tests to verify the validity of the models. Although initial simulations proved promising, more complex models resulted in large discrepancies between projections and actual results.

Full Fin - Oomoo

The first set of prototypes was modeled as a small scale full fin cast in silicone. Oomoo was the silicone chosen for the properties of 240 psi tensile strength and 250% elongation at break. The mold was created as a two part model in SolidWorks then 3D printed. The size of the mold allowed for a fin that was approximately ¼ scale of the final design choice.

The full fin mold had an open top to allow polystyrene foam channel inserts to be suspended in the mold and for the uncured silicone to be poured. The foam was carved by hand into two half cylinders and then wrapped in string to create strategic inextensibility as shown by the Fiber Reinforced Actuators in the Soft Robotics Toolkit. The string wrapped around the outside of the foam was to prevent the channel from expanding outward in all directions, constraining deformations to the axial direction. Another set of strings lying against the flat face of the half cylinder in the root-to-tip direction created the inextensible layer across the chord line of the fin that would cause it to curl rather than stretch linearly. For this to actually be implemented, the foam channels were set in the silicone then melted out with acetone. The strings on the outside of the foam stayed embedded in the silicone to provide the structure for the movement once the channels were filled with fluid. The two channels for the small scale fin were placed on either side of a laterally oriented center plane to allow movement in both directions depending on which channel was filled.


 

Two Channel Full Fin Mold and Oomoo Positive

To test it, a manual bike pump was attached to the inlet hole of one channel and pressurized air was pumped in. This prototype fin failed to actuate because the channels did not provide sufficient area for the length and thickness of the silicone.

Another small full fin was cast with the same foam and string concept, but one large channel was created with the intent of decreasing the amount of silicone that needed to be actuated. The channel followed the same shape as the fin, keeping a consistent edge size. The inextensible string was along one side, which meant the fin would theoretically only bend in one direction when filled air.

There was a minimal amount of actuation when this was tested because the silicone proved to be too thick. Cutting down the edges improved deflection slightly, but after some more testing the strings began to fall out of the silicone. They were unable to be completely embedded in the silicone during the curing because of the way it was laid against the foam. This method was determined to be unreliable and inexact in implementation, so other methods were pursued.

Half Fin - Oomoo

The next method used to create the fin drew inspiration from the manufacturing techniques used for the WPI Soft Robotic Fish. Instead of making one mold for one fin, the fin would be constructed of two symmetrical halves with an inextensible layer between them. Rather than embedding polystyrene or other materials for the channels fully in the silicone, the mold for half the fin utilized a panel that molded the channels directly into the silicone. This allowed for more precisely made channels and consistent models.

The new printed mold was slightly larger than the first, but maintained the shape and proportions of the first small scale fin, which had been modeled extensively in SolidWorks. The set of channels was designed with one main channel running from the fin root toward the tip of the fin. Four perpendicular channels were spaced equally across approximately one third of the fin. This way the outer two thirds of the fin remained passive, allowing for more natural motion. Additionally, this passive tip was intended to create beneficial vortices, providing greater thrust. The channels were half cylinder shaped with a constant radius. The widths of the channels across the fin varied in an attempt to keep the distances to the leading and trailing edges the same.

An inextensible layer was created separately by laying a section of tulle mesh on a flat surface and pouring silicone over it. The mesh allowed the inextensible layer to bend and twist, but not stretch in any direction. Once the silicone half fin was set, the two parts were bonded together with more silicone. A small opening to the main channel allowed the connection to the bike pump.

The first test was promising, however, very quickly the bond around the channels started to fail. This resulted in essentially one large channel, but there was still some actuation in one direction which indicated that the inextensible layer was functioning properly. Attempts to cut open the fin and fix the bond around the channels with high strength adhesive failed, but did prove that super glue does not adhere well to silicone.

Half Fin - Dragon Skin

Once the Oomoo had been used up, the decision was made to switch to Dragon Skin 10 for its increased compressibility, flexibility and elasticity. Dragon Skin 10 has a tensile strength of 475 psi, over twice that of Oomoo, and 1000% elongation at break. The same half mold was cast once more with the new silicone. Using the old inextensible layer did not work because Oomoo does not bond to Dragon Skin. A new inextensible layer was created the same way as before, but with the Dragon Skin, and the two parts were bonded together.

Dragon Skin Upper Fin Section

During the bonding process, some of the channels filled in with the extra silicone, but there was still significant performance improvement using the Dragon Skin over the Oomoo. The material properties of the Dragon Skin allowed for more deflection from the fin root to tip. Because of the increased compressibility and elasticity, the fin was able to bend much more easily. Once again, after repeated testing the bonding between the channel walls and the inextensible layer started to weaken and eventually failed, separating the layers

Comparison of SolidWorks Simulation with Actual Results

Dragon Skin Half-Fin Iteration 1

A new design was simulated in SolidWorks for the channel configuration of the same fin. A simple static simulation was conducted. The data sheet for the DragonSkin did not include all of the material properties required, so certain aspects needed to be derived. To complete a full half wing and allow it to actuate, a very thin layer was added to close the channels. This layer had the material properties of Delrin acetal plastic, which is flexible but inelastic. The thick end of the fin was fixed and a pressure of 50 psi was added uniformly to the inner surfaces of the channels and areas of the inextensible layer that were covering the channels.

The semi-cylindrical channels were replaced with rectangular channels of varying depths such that their distances from the top of the fin were each the same. They were also much thinner and closer together. The simulations showed that this allowed more actuation with less bulging because the forces were more equally distributed and there was less material per channel to deform.

To try to avoid issues with the bonding failing between the fin and the inextensible layer, the silicone was poured onto the mesh and the fin was placed immediately on top of it. The intention was that the new silicone would bond well to the fin rather than depending on a very small amount of silicone to bond two separate pieces. With this method, channel loss was significantly reduced; only the last, smallest channel filled in.

This prototype was the most successful of the small scale tests with almost 90 degrees of actuation. There were a few places where the fin expanded perpendicular to the desired direction, with bulges where the material was thinner, which shows the consistency needs to be increased in future prototypes. With this improvement, this design was chosen for the final design.

While the 90 degrees of actuation was promising for the project, this was a result that deviated a significant from the SolidWorks simulation. This was most likely due to a more complex model than the previous fin and over-simplistic choice of simulation. In addition, the derived material properties for the DragonSkin represent a potential source for error. The confluence of factors likely resulted in the inaccurate simulation. Given the unreliability, the decision was made to discontinue simulating the models.

 

Comparison of SolidWorks Simulation with Actual Results

Dragon Skin Half-Fin Iteration 2

Fin Manufacturing Process

Due to limitations on rapid prototype molding, namely the build area of available 3D printers (approximately 10in x 6in with the Makerbot 2), it was not an appropriate production method for larger fins. Therefore, in order to create an 18in fin, a new production method had to be devised. In order to save on materials costs for testing iterations, before going directly to full scale from the small scale, it was decided to develop production methods with a medium scale fin.

In order to make a negative mold of an object, a positive is required. A small test was done with the high density foam, previously used to create channels, to see how difficult it would be to carve a fin positive. Two pieces of foam were glued together to create a cube. From this point, using knives and rasps, a rough 6in foam fin was shaped by hand.

Once the positive was created, plaster was mixed as the mold material. Plaster is inexpensive, cures quickly and is a common material for making reusable molds. As proof of concept, the foam fin was suspended in a paper cup and the plaster was poured around it. Once set, the plaster block was cut in half to remove the fin.

Small Scale Plaster Test

This process demonstrated that plaster could be used for a mold for a larger scale design. There were, however, a few things that needed to be improved. For instance, cutting the mold introduced potential for failure, as the plaster was prone to crumbling. To avoid cutting, the decision was made that the molds would be created for one half of the fin at a time, similar to the methodology employed for the small scale mold. Another place for improvement was the interface between the foam and the plaster. The foam was slightly pitted and porous, creating an imperfect mold.

With this new knowledge, it was decided the medium scale fin would be more feasible as the final prototype. This was based on the manufacturing time expected with each fin pour, and the cost of volume of silicone needed for the project.

Mechanical System Design

Design Considerations

In order to ensure the fin could be actuated, a target operating depth was determined. Using the generalization that every 10 meters of ocean depth causes a pressure increase of approximately 1 atm, the hydraulic system was designed to operate at depths of up to 20 meters. This generalization assumes water is perfectly incompressible, which is a reasonable simplification at this depth. At 1000m depth, water compresses less than one percent.

Furthermore, research showed that coastal rays live between 0 and 80 feet (0 and 24 meters), spending the majority of their life between 30 and 35 feet (9 to 10.5 meters). This reinforces the choice of target depth. Most coral reefs are less than 150 feet (45 meters) deep, although some extend as deep at 400 feet (122 meters). The current design would require an internal pressure of over 13 atm to safely operate at the 400 ft depth. While this is possible with the correct equipment, limited project budget and time frame make this impractical.

Hydraulic System Layout

A hydraulic system utilizes fluidic pressure to create motion. The hydraulic system is divided into two major loops. The high pressure side feeds from the pump outlet into each valve. From there, the valve can be opened to allow the working fluid under high pressure to enter the fin channel. When the valve closes, the fluid flows out of the fin into the low pressure line. The low pressure line feeds to the pump and the process is repeated.


Plumbing Layout

Working Fluid

A hydraulic system requires a working fluid, or any fluid that is contained in the system. This fluid is pressurized, which is translated to mechanical energy through an actuator. The requirements for an ideal working fluid are that the fluid be incompressible, non-combustible, easily contained and with a constant viscosity. An additional consideration for this project was potential environmental impact. This leaves two common choices for working fluids; water and vegetable oil. Water was chosen for this prototype in order to ensure ease of cleanup and availability of parts. Potential risks associated with water as a working fluid include increased cavitation risk and narrower range of operating temperature.

Pump

The pump is what creates pressure in a fluidic system. The pump for this project needed to deliver enough pressure for the robot to operate at the target depth, and be rated for usage with the working fluid. By choosing water, there was no need for a specialty pump, reducing both the price and lead time. The initial choice was a Flojet Demand Pump 1.6 GPM 12V DC 60 psi (FJC-PMP-D3131H5011A).

Flojet Pump

The deciding factor in purchasing the pump was how much pressure it could provide. The Flojet pump 60 psi (4.1 atm) allowed for a significant safety margin. Running the pump at maximum pressure constantly can cause early wearing of parts and seals. This model can also run dry for short periods, ensuring the pump will not be damaged by a system leak. Electrical power draw is an important factor for an AUV, limiting choices of pumps further. The initial pump chosen requires 12V DC and draws 7A.

As the largest source of power draw in the project, the pump was a topic of continued research. Clark Solutions in Hudson, MA offered an educational discount on their MG200 gear pump, allowing greater pressure with less overall energy consumption. The MG200 provides up to 290 psi (20 atm) of pressure while requiring 12V DC and drawing 3.4A peak. This resulted in about half the power draw of the Flojet pump, while also being a significantly smaller, lighter, and quieter pump.

MG200 Gear Pump

Valves

In order to control the pressurization and depressurization of the fin channels, controllable valves are necessary. The options were solenoid valves and motorized valves. Solenoid valves remain in a default state, either open or closed, and require a continuous energy draw to switch and hold the non-default state. Motorized valves require an energy draw to open or close, but can remain in that state with no draw. Solenoid valves are generally smaller and respond more quickly than motorized valves. The desired characteristics of the valve included a low current draw, rated for 60+ psi, and the ability to run off 12V DC. For this system design, a three way valve is required, further reducing valve options. The initial choice was the Misol 3 way motorized ball valve (DN15) due to its low price, availability, 800mA current draw, and ability to fit all other requirements.

3-Way Valve

Once one of these valves was acquired, it became clear a replacement would be necessary. While some small scale tests proved that the valve worked in the desired fashion, it was too large and heavy when considering twelve valves were needed. The time to switch the direction of the valve also introduced a delay between the desired change and the actual change. The results of these tests lead to the decision to acquire solenoid valves.

The valves used were the drip irrigation 3 way 12V solenoid valve from Ningbo Yaofeng Hydraulic Electrics Company. They are rated for a range of 9 to 150 psi and use 290 mA while holding.

Solenoid Valve

Electrical System Design

The electrical components of the robot depend heavily on requirements set by the other systems; motor voltage, current draw, sensors necessary for an intelligent control system, and size and weight restrictions all factor into the selection of parts. The electrical system collects data with sensors and outputs the power that drives the other systems. It allows the microcontroller to exert control on the rest of the robot, so it can be roughly equated to the robot’s nervous system.

Microcontroller

The microcontroller is the brain of the robot, taking in data, performing computations, and controlling the actuators. There were several criteria important to choosing a microcontroller, such as digital I/O, available communication protocols, reliability and support. The ARM Cortex-M4 microprocessor was a favorable solution. The Cortex-M series is known for optimization of low cost, high performance, and low power. The M4F specifically is a 32-bit processor with a floating point unit (FPU). This is helpful for the calculations required in dealing with various sensors.

Once the microprocessor was chosen, the microcontroller had to be picked. The top choices were the MSP432, STM32 L4 and AVR UC3 C-Series. The MSP432 was the final decision based on the features, including a 24-channel 14-bit analog to digital converter (ADC), 6 timers, UART, I2C and SPI communication interfaces, and a JTAG interface for debugging. For prototyping, the MSP432 is conveniently available in a development board, which features breakout pins, LEDs, switches, and available BoosterPacks for further functionality.

MSP-EXP432P401R (MSP432 Launchpad)

Sensors

Sensors are the eyes and ears of the robot, providing input for the microcontroller. The sensors are split up into two categories: essential and payload. Essential sensors are used for determining orientation, depth, and other important information about the robot’s state. Payload sensors are optional, used for data collection of the environment or other mission specific information. For this prototype, there are no payload sensors, but in the future they could be integrated with reasonable ease.

Below is the decision matrix for selecting the essential pressure sensor.

 

Weight

Barometric Pressure Sensor

Submersible Pressure Transducer

MEMS Pressure Sensor

Resolution

2

2

1

1

Range

2

-1

1

2

Power Requirements

2

1

-1

1

Size

1

2

0

2

Cost

1

2

1

1

Unweighted Totals

6

2

7

Weighted Totals

8

3

11

Pressure Sensor Characteristics

Inertial Measurement Unit

MPU-9250

The first essential sensor board is the inertial measurement unit (IMU). This is a chip common for consumer electronics equipment, such as smartphones and wearable sensors, due to motion processing and MotionFusion algorithms.5 The MPU-9150 was originally chosen for its 9-axis measurement from a 3-axis accelerometer, gyroscope and magnetometer. A breakout board was available from SparkFun with standard header pins for an easy interface. This chip was later replaced with the MPU-9250 - a newer model of the now-deprecated MPU-9150.

This board communicates with the MSP432 via I2C. The MSP432 sends initial configuration information on powerup. Afterwards, certain registers on the board contain the continuously updated information for each axis of each sensor. The MSP432 then only has to send a request detailing the appropriate register, specified in the datasheet, and wait to receive the information which can be stored and used for processing. These values provide information about orientation that can applied to the control system.

Pressure Sensor

MS5803-14BA Pressure Sensor

The second essential sensor is the pressure sensor that monitors the robot’s depth. The MS5803-14BA was selected due to its wide range, high precision, and availability on a breakout board from SparkFun for ease of use. This sensor is instrumental in giving the robot information about its position and developing failsafe systems.

There are two communication protocols available with this board: I2C and SPI. While SPI can transfer data at a higher rate, it requires more pins than I2C. For this application, high speed sensor updates are not required. I2C was chosen both for these reasons and to match the IMU. The way I2C works, there can be one master with several slaves on the same bus. The MSP432 functions as the master and can continuously switch between the two sensors on the I2C bus to update the information being processed.

An advantage of getting this sensor on the breakout board is the inclusion of an ADC. The MSP432 can send a request to the sensor board to convert the most recent value, wait a short time for the conversion to happen, then receive the updated sensor value. The result comes in a integer that can be converted into the pressure in millibar. The calculations require several factory calibrated values that are stored in the PROM of the chip. These constants are specific to each sensor and are easily retrieved by reading specific registers.

Power Systems

Batteries

The robot has two major systems requiring power. The first system is the propulsion, requiring a 12V battery, with significant current draw. This draws power only when the robot is pressurizing the hydraulic system or flapping.The second system, which consists of the microcontroller and sensors, requires 5V. The battery needed to be energy dense, rechargeable, stable long term, non-combustible, and environmentally friendly.7 With these criteria, two types of batteries were considered for selection: lithium polymer (LiPo) and lithium iron phosphate (LiFePo4).

LiPo batteries were considered for their high energy density and commercial availability. LiPo batteries are commonly used and readily available for purchase.This eliminated any wait time on a custom order or risk of backordering. LiPo batteries are the lightest lithium battery, while still maintaining a high energy density, ensuring no unnecessary weight is added to the system. These batteries have a long life, are environmentally friendly, and are much safer than traditional lead-acid batteries. LiPo batteries are not perfectly charge stable, potentially exposing sensitive equipment to voltage fluctuations. The pump and valve system is capable of withstanding small disturbances, however the sensors and microcontrollers could be susceptible to damage.

LiFePo4 batteries, a type of lithium ion battery, were a promising option due to their increased performance when compared to other lithium polymer batteries. Being the safest lithium battery, they does not pose a risk to safety or the environment, and maintain the highest energy density of all lithium batteries to date. Additionally, LiFePo4 batteries have a minimum life span of 3 years from production date.

LiFePo4 Battery

When actually purchasing the batteries, a small, light 12V LiFePo4 battery was found, with an included charging circuit, for a relatively low price. At 1.76lb and almost 50 in3, this battery can fit easily in the hull without excessively disturbing the weight distribution.

At this point, rather than purchase a separate battery for the electronics, it was discovered that a voltage regulator would introduce significant price savings without significant energy loss. The LT1085 voltage regulator is capable of shifting 12V to a stable 5V, with the implementation of some capacitors for stability. It is a relatively efficient chip that can handle 3A, which is enough current for all of the electronics. The other benefit of the voltage regulator is that the 5V output remains stable, preventing damage to the electronics from fluctuations.

Motor Drivers

In order to control the valves while providing the amount of power needed to switch and hold valve states, several L2930 H-Bridge motor drivers are used. This H-Bridge integrated chip takes an input signal from the microcontroller and adjusts the voltage of its output pins accordingly. The power output by the chip comes from battery power connected directly to the chip and is therefore not limited by the current ratings of the microcontroller. Each motor driver chip has four inputs and outputs, enough for four valves. However, for organizational purposes, four chips are used with three valves each so that every half-fin is attached its own motor driver.

The gear pump draws a steady 3.5A, which is more than the L2930s can provide, so a different motor driver chip had to be selected. The Pololu G2 High-Power Motor Driver 24v13 can handle 6.5-40V, well over what would be required for a safety margin for ripple voltage on the supply line. The continuous current capacity for the motor driver is rated at 13A, allowing for an inrush current of almost 400% of nominal current consumption.

Pololu G2 High-Power Motor Driver 24v13

Overall System Design

System Block Diagram

Hull Design & Waterproofing

Ensuring the electrical and mechanical systems are protected from the water is vital for the survival of the prototype in an aquatic environment. As such, much consideration was put into the the robot’s hull and waterproofing. All electronic components are housed within a smaller enclosure that is water resistant independent of the primary hull. The internal enclosure acts as a second defense should the primary hull leak. The plan was to buy a commercially available pressure vessel to ensure its success. This hull was planned to be rounded into a hydrodynamic shape by use of silicone. The primary hull had an access panel to allow for modifications and repairs to the internal components.

System Controls

Motion through the water is theoretically achieved by actuating each of the fin segments in sequence in order, creating sinusoidal motion. Each section is raised by both pressurizing the low channel set and draining the high channel set. Conversely, a section is lowered by pressurizing the high channel set, and draining the low channel set. The valves connect each partition to either high pressure when unpowered, or low pressure when the valve is powered, thereby filling, or draining its channel set. By powering either the top or the bottom exclusively, and sequencing these top/bottom pairs from front to back, the fin may achieve sinusoidal motion.

In order to help achieve desirable movement, the robot’s two sensors provide feedback about its pose in the water. The information from the IMU and pressure sensor is fed into the robot’s microcontroller, the MSP432. A PID control prototype was developed to regulate the frequency of oscillation to move the robot around a horizontal plane.

Pinout Diagram

Shown above is a pinout diagram of the MSP432 and peripheral electrical systems. The sensors on the right from top to bottom are the IMU and the pressure sensor. The SDA pins on both sensors are connected together and to the pin P1.6 on the MSP432. Likewise, the SCL pins are connected together and to P1.7. These form the I2C bus. SDA is the data line (blue), where information can be sent in either direction. For the purpose of this project, the MSP432 is the master, sending requests for information to the sensors. The sensors are the slaves, sending gathered data back to the MSP432 for processing. The SCL line (green) is for the clock. It is important that these are connected together and synchronized because bits are sent on clock cycles.

In order to control which sensor the MSP432 is communicating with, it needs to send the address of the slave with the start condition. Each sensor has an address as described in its datasheet. For instance, the pressure sensor has the address 0b0111011n, where n is replaced with a 1 if the CSB pin is pulled high and 0 if the CSB pin is pulled low. This theoretically allows two of the same sensors to be placed on one bus. In this configuration, the pin is pulled high, so the address is 0b01110111. The IMU has a similar situation by pulling AD0 high, setting the LSB of the address to 1.

The PS pin on the pressure sensor is pulled high to select I2C mode, because it is also capable of communicating with SPI. The decision to communicate with this sensor using I2C was made to keep the communication protocol consistent with the IMU, which is only capable of I2C.

To the left of the MSP is the H-bridge motor driver circuit. Motor power (dark red) is provided by a 12V battery, and chip power (chip enable) is provided by the MSP (light red). Ground for the circuit (grey) is connected to both the MSP ground and ground of the battery. By providing a PWM signal from P4.0 and P4.1 (blue), the voltage level at the adjacent output pins (green) can be controlled. With no input signal, the corresponding output pin will be grounded, and with a 100% duty cycle (a high pin) input, the output will be motor power (12 V). By driving one input pin low and the other high, a voltage difference of 12 V in one direction is created across the valve’s power cables. The direction can be switched by swapping which input pin is high and which is low. By this method, the valve can be driven in either direction at the command of the microcontroller. By mirroring the input, output, and ground pins across the H-bridge, and connecting them to different input pins, another valve, and the same ground rail (grey) respectively, a second valve can be driven through the same motor driver chip. In order for this to work, the top-left pin, which serves as the second half’s motor enable, must be connected to the 5 V (light red) rail.

Design Limitations

Common silicone exists that can withstand up to 1200 psi (80 atm) allowing a depth of almost a half mile (800 meters), proving the feasibility of a deeper diving model. It is worth noting this design is not feasible for extreme depth, as the average ocean depth is 2.3 miles (3700 meters) and would require over 370 atm of pressure to reach. The limitations on flexible enough materials make this a current impossibility.

Projected Budget

In order to better organize the project, an estimated budget was established. The following figures are rounded up and account for shipping, taxes, and has anticipated extra required parts. Most of the needed tools and equipment were provided by WPI or team members’ personal supplies. For this reason, tool costs were not included in the budget.

Material

Description

Units Used

Cost per Unit

Total Cost

Silicon for Molding

material for molding fins

1

$200.00

$200.00

Hull Casing

pressure housing for components

1

$500.00

$500.00

Pump

pressurization of fluid for actuation

1

$120.00

$120.00

Valves

automated valves to control actuation

12

$25.00

$300.00

Reservoir Tank

tank to hold excess hydraulic fluid

1

$10.00

$10.00

PVC Piping

piping & fittings used for pressure system (ft)

20

$2.50

$50.00

Hydraulic Fluid

working fluid in our system

1

$20.00

$20.00

Large Battery

power for the pump and valves

1

$60.00

$60.00

Small Battery

power for the micro-controller

1

$20.00

$20.00

Large Motor Driver Circuit

system to power pump

1

$30.00

$30.00

Small Motor Driver Circuit

system to power 2 valves

6

$5.00

$30.00

Microcontroller

"the brain" of the robot

1

$25.00

$25.00

IMU

sensor for navigation in water

1

$30.00

$30.00

Pressure Sensor

sensor for safe depth detection

1

$65.00

$65.00

Other ECE Components

misc parts needed for interfacing

1

$40.00

$40.00

Project Total: $1,500.00